• Clinical science

Arterial blood gas analysis and pulse oximetry

Abstract

Arterial blood gas (ABG) analysis is a test regularly performed to measure oxygen saturation, carbon dioxide, and bicarbonate blood levels. It provides quick assessment of gas exchange processes and acid-base balance. Pulse oximetry is a non-invasive and quick way of measuring the oxygen saturation of peripheral arterial hemoglobin. The test relies on the fact that oxygenated and deoxygenated hemoglobin absorb different wavelengths of light. Physiological levels of oxygen saturation are generally above 95%.These tests provide vital information about a patient that is especially important in emergency and intensive care settings.

Pulse oximetry

  • Technical background
    • Oxygenated hemoglobin (O2Hb) and deoxygenated hemoglobin (HHb) exhibit different properties of light absorption
      • O2Hb: ↑ infrared light absorption, allows ↑ red light pass through the measurement site (e.g., fingertip)
      • HHb: ↑ red light absorption, allows ↑ infrared light pass through the measurement site
    • An oximeter uses LEDs (light-emitting diodes) emitting both red and infrared light → a photodetector is positioned on the other side of the finger, opposite the LEDs, and detects the amount of light (and whether it is red or infrared light) passing through the measurement site → a processing unit calculates the amount of O2Hb → oximeter displays SpO2
  • Reference range: Resting SpO2 > 95% is considered normal.
    • A PaO2 of 100 mm Hg is necessary to reach a SpO2 level of ∼ 98%.

Pulse oximetry provides falsely high values in cases of carbon monoxide poisoning, as complexes of hemoglobin and carbon monoxide are indistinguishable from oxygen-hemoglobin complexes!

References:[1][2][3][4]

Arterial blood gas analysis

  • Measured parameters
  • Reference ranges
    • PaCO2: 35–45 mm Hg (4.7–6 kPa)
    • pH: 7.35–7.45
    • HCO3-: 21 to 27 mEq/L
    • Base excess: -2 to +3 mmol/L
    • Resting PaO2 > 80 mm Hg is considered normal.
  • Procedure:
    • A modified Allen test must be performed before the radial artery is punctured to assess collateral circulation in the hand.
      1. Manual pressure below the wrist is applied to both arteries, the hand is elevated, and the patient is asked to clench their fist for about 30 seconds to induce transient ischemia, leading to pallor of the hand.
      2. When the pressure is released from the ulnar artery, the patient's hand rapidly returns to normal color if collateral circulation is present.
      3. Interpretation: The test result is normal if the patient's hand rapidly returns to normal color. The test is negative and considered pathological if the patient's hand remains at least partially pale.
    • Arterial blood can be drawn from radial arteries or an indwelling arterial catheter.
  • Interpretation

References:[4][5]

Mixed oxygen venous saturation